1. Field
This disclosure relates generally to communication systems and equipment, and more specifically, to techniques and apparatus for adaptive filtering, and adaptive filtering in an echo reduction system in a communication system.
2. Related Art
An adaptive filter is a filter having a transfer function that is adjusted according to an adaptation algorithm. Because of the complexity of the adaptation algorithms, many adaptive filters use digital filters and digital signal processing to adapt the filter transfer function based on, for example, the input signals.
For some applications, such as echo cancellation, adaptive filters can be used since some parameters of the desired processing operation (for instance, the properties of the system (or circuit) that produces the echo signal) are not known in advance.
Echo can have a major effect on voice quality in telecommunication networks (such as the Public Switching Telephone Network (PSTN) or Packet Telephony (PT) network). The objectionable effect of echo results from a combination of reflections from network components such as two- to four-wire converters (e.g., an impedance mismatch of a hybrid circuit, which is a device used to convert signals from a four-wire communication network interface to a two-wire local subscriber loop, and vice versa), together with signal processing and transmission delay. Echo may cause users difficulty in talking or listening over a telephone connection, and it may also affect the transmission of voiceband data, fax and text.
Echo cancellation can be used in a telecommunications network to ensure voice quality through elimination or reduction of electric or line echo from the telecommunications network. Echoes develop, or are created, in an “echo path,” which is a system that includes all transmission facilities and equipment (including the hybrid circuit, the subscriber loop, and terminating telephone set) connected to the near end of an echo canceller.
An echo canceller is a device that can use adaptive signal processing to reduce or eliminate echoes to allow successful transmission of voice and/or voiceband data (such as modem and facsimile signals). Echo cancellers can be placed in the four-wire portion of a circuit, and reduce (or cancel) the echo by subtracting an estimate of the echo from the near end signal that includes the echo signal. For a more detailed discussion of echo cancellers, see the document entitled “Digital Network Echo Cancellers,” which is published by The International Telecommunication Union's (ITU) Telecommunication Standardization Sector (ITU-T) as ITU-T Recommendation G.168.
One characteristic of a good echo canceller is rapid convergence. Convergence can be generally defined as the time the echo canceller needs to produce an estimate of an echo signal and reduce the echo signal below a threshold. In some echo canceller embodiments, multiple adaptive filters can be used, wherein results or data from a first adaptive filter can be used as an input to a second adapter filter. In such an echo canceller, faster filter adaptation in the first adaptive filter can improve performance of the overall echo canceller.